Some times you click the wrong button, or the customer changes her mind. This happens.
In DX, you can easily edit the active order or change the items in the order.
Simply click on the Pencil icon in the top right of the Order Summary.
You will then be able to remove or increase any ticket categories.
If you increase or decrease the number of tickets for a screening with seat selection, you will be prompted to remove or select additional seats during this process.
And remember - you can always start over with a fresh order by clicking on Cancel order at the bottom of the Order Summary.